Muscle glycogen phosphorylase, presumably inactive in the resting state, is activated by phosphorylase kinase and inactivated by phosphorylase phosphatase. In mice of the I strain, the kinase is almost entirely absent from skeletal muscle, yet glycogen breakdown proceeds following epinephrine or electrical stimulation. In these mice activation may occur by a binding of phosphorylase with 5'-AMP since this activation takes place in vitro. The mechanism of this process and the counter process of inactivation will be studied in skeletal muscle of the I strain mice, and comparative studies will be made in C57 strain mice where phosphorylase kinase is present in quantity. Crosses of these two strains will also provide further source material since the gene for the kinase is known to be on the X chromosome, and as well, the gene(s) that control the concentration of glycogen in the resting muscle.